But Supreme Court justice could be a 'cushier' job than President. Potentially a life term and you don't criticized so much for not going to work every day. So it's improbable that Thomas could be convinced to run.
And Thomas wouldn't be both President and Supreme Court justice simultaneously.
I see no particular requirement under the Constitution for Thomas to resign from the Court in order to run for President.
A sitting justice as a candidte was not uncommon during the 19th century, and I believe it last occurred in 1916, when Charles Evans Hughes was the GOP nominee. He resigned from the court to run in the general election, but, interestingly, only after the convention nominated him.
Nobody at the time seemed to see any constitutional problem.